This invention relates to polymeric plexifilamentary film-fibril strands. More particularly, the invention relates to improvements in the process for flash-spinning and laying down polymeric plexifilamentary film-fibril strands.
In the process for making flash-spun fibers, as disclosed in U.S. Pat. No. 3,081,519 to Blades et al. (assigned to DuPont), a solution of fiber-forming polymer in a liquid spin agent that is not a solvent for the polymer below the liquid""s normal boiling point, is maintained at a temperature above the normal boiling point of the liquid and at autogenous pressure or greater, and is then spun into a zone of lower temperature and substantially lower pressure to generate plexifilamentary film-fibril strands. Suitable spin agents are described and include aromatic hydrocarbons such as benzene and toluene, aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, and their isomers and homologs; alicyclic hydrocarbons such as cyclohexane; unsaturated hydrocarbons; and halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, and methyl chloride. The Blades et al. patent does not describe formation of flash-spun sheets. As disclosed in U.S. Pat. No. 3,227,794 to Anderson et al. (assigned to DuPont), the solution flash-spinning process requires a spin agent that: (1) is a non-solvent to the polymer below the spin agent""s normal boiling point; (2) forms a solution with the polymer at high pressure; (3) forms a desired two-phase dispersion with the polymer when the solution pressure is reduced slightly in a letdown chamber; and (4) flash vaporizes when released from the letdown chamber into a zone of substantially lower pressure.
The flash-spinning process normally includes a step of applying an electrostatic charge to a flattened and partially spread web of plexifilamentary film-fibril strands after the web is spun from a spin orifice and before it is laid down on a grounded moving belt to form a sheet. The electrostatic charge is applied by passing the web through a corona field created between a multi-needle ion gun and a grounded target plate. When the web passes through the corona field, it picks up charged particles migrating from the ion gun to the target plate. The electrostatic charges applied to the individual fibrils of the web cause the fibrils to repel one another, thus separating the fibrils and further xe2x80x9copening-upxe2x80x9d the film-fibril web. Each charged web is then laid down, along with other webs from adjacent spin packs onto the moving belt. Because the webs are charged, they are first attracted to the grounded moving belt and once laid down, they remain pinned in place on the belt.
During the flash-spinning process, it is important that the charge density on the webs not exceed a value that leads to electrical breakdown of the gaseous atmosphere in the spin cell, which would cause arcing between the webs and belt. When arcing occurs, the webs lose their charge and the pinning forces between the webs and the belt may be reduced such that the webs do not remain pinned to the belt. When the webs are not properly pinned to the belt, the webs may be pulled and moved by the stream of gaseous spin fluid. This causes the webs to roll into bundles such that the sheet produced from the webs is not uniform and contains defects.
Commercial sheet products made from polyethylene plexifilamentary film-fibril strands were historically produced by flash-spinning a spin fluid comprised of polyethylene in a perchlorofluorocarbon (CFC) spin agent such as trichlorofluoromethane. Unfortunately, CFCs are considered to be stratospheric ozone depletion chemicals. Alternative compounds suitable for the flash-spinning process have been developed that do not cause ozone depletion or contribute to global warming.
Included among the alternate spin agents that have been investigated are saturated hydrocarbons such as n-pentane. Although saturated hydrocarbons are not ozone depleting, they have the disadvantage, as compared to CFCs, of reducing the effective electrostatic charge applied to the flash-spun web as the web passes through the electrostatic field for a given current. As a result, the webs are not as fully opened up and the resulting non-woven sheet is less uniform than a sheet formed of more fully charged webs. In addition, saturated hydrocarbon gases tend to have low breakdown strengths. When the charge density on the web exceeds the gas""s ability to support it, a conductive path forms through the gas, which is seen as an arc. The arc bleeds charge off the fibrils of the web, resulting in poor lay-down on the collection belt. The low breakdown strength of a saturated hydrocarbon gas requires a reduction in the rate at which the fibers can be processed (reduced polymer flow rate to the process) compared to spin agents having higher breakdown strength such as CFC""s.
U.S. Pat. No. 5,643,525 issued to McGinty et al. describes a method for improving polyolefin web charging during flash-spinning in which the electrostatic charging step is conducted in an atmosphere comprising at least one charge-improving compound. The charge improving compounds can be introduced at very low concentrations as a gas, vapor, or mist, directly into the electrostatic charging atmosphere in the spin cell. The charge-improving compounds are substances which when ionized in the corona charging zone form stable, slow moving ions. The presence of these ions creates a more stable corona, which increases the amount of charge that can be applied to the web compared to the charge that would be achieved in the absence of the charge-improving compound. Although this approach has been found to be effective in increasing the charge on the web, the charge-improving additives do not affect the overall properties of the gaseous atmosphere in the spin cell unless the charge improving compounds are used at concentrations far higher than is possible in a flash-spinning process. Thus, arcing between the webs being laid down and the belt remains a problem. It is therefore desirable to develop a non-ozone depleting spin agent for use with existing flash spinning equipment that greatly reduces or eliminates arcing between the laid down webs and the belt so as to improve the uniformity of sheet produced from the webs laid down on the grounded belt.
The present invention is directed to a process for flash-spinning a web of plexifilamentary film-fibril strands of synthetic fiber-forming polymer and laying down the web to form a nonwoven batt material therefrom. The process includes the the step of generating a spin fluid consisting essentially of synthetic fiber-forming polymer and a spin agent, wherein the spin agent is comprised of at least 80% by weight, based on the total weight of the spin agent, of hydrocarbons comprised substantially exclusively of carbon and hydrogen atoms. The hydrocarbons are comprised of at least 25% by weight of unsaturated hydrocarbons having 4-8 carbon atoms. The process further includes the steps of flash-spinning the spin fluid at a pressure that is greater than the autogenous pressure of the spin fluid into a spin cell maintained at lower pressure to form a web of plexifilamentary film-fibril strands of said synthetic fiber-forming polymer, applying an electrostatic charge to the web by passing the web through an electric corona, and laying the web onto a grounded surface to form a batt of plexifilamentary film-fibril strands that is suitable for being consolidated into a sheet.
The spin fluid in the process of the invention is preferably comprised of between 5 and 30 weight percent, based on the total weight of the spin fluid, of a fiber-forming polymer. Preferably, the fiber forming polymer is a polyolefin such as polyethylene or polypropylene.
The unsaturated hydrocarbons in the spin agent are preferably selected from the group of alkenes having the formula CnH2n and cycloalkenes having the formula CnH2nxe2x88x922, where n equals 4, 5, 6, 7, or 8. The spin agent has an atmospheric boiling point between 15xc2x0 C. and 100xc2x0 C. Preferably, the unsaturated hydrocarbon is selected from the group of 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, and their structural isomers, or from the group of cyclobutene, cyclopentene, cyclohexene, cycloheptene, and cyclooctene. According to a more preferred embodiment of the invention, the spin agent is comprised of at least 90% by weight of unsaturated hydrocarbons selected from the group of alkenes having the formula CnH2n and cycloalkenes having the formula CnH2nxe2x88x922, where n equals 4, 5, 6, 7, or 8. More preferably, the spin agent consists essentially of unsaturated hydrocarbons selected from the group of alkenes having the formula CnH2n and cycloalkenes having the formula CnH2nxe2x88x922, where n equals 4, 5, 6, 7, or 8. Most preferably, the spin agent consists essentially of unsaturated hydrocarbons selected from the group of 1-pentene, 1-hexene, and their structural isomers. According to one preferred embodiment of the invention, the spin agent consists essentially of hydrocarbons comprised substantially exclusively of carbon and hydrogen atoms. According to an alternative embodiment of the invention, the spin agent may include at least 10% by weight of saturated hydrocarbons selected from the group of alkanes having the formula CnH2n+2, and cycloparaffins having the formula CnH2n, where n=4, 5, 6, 7, or 8.
According to the preferred process of the invention, the process for flash-spinning a web of plexifilamentary film-fibril strands of synthetic fiber-forming polyolefin and laying down the web to form a nonwoven sheet material therefrom, comprises the steps of generating a spin fluid consisting essentially of 5 to 35 weight percent, based on the total weight of the spin fluid, of synthetic fiber-forming polyolefin and a spin agent, flash-spinning the spin fluid at a pressure that is greater than the autogenous pressure of the spin fluid into a spin cell maintained at lower pressure to form a web of plexifilamentary film-fibril strands of the synthetic fiber-forming polyolefin, applying an electrostatic charge to the web by passing the web through an electric corona, laying the web onto a grounded surface to form the web into a fibrous batt, consolidating said fibrous batt to form a fibrous nonwoven sheet, and removing the fibrous nonwoven sheet from the spin cell. The spin agent is comprised of at least 90% by weight, based on the total weight of the spin agent, of hydrocarbons comprised substantially exclusively of carbon and hydrogen atoms, and the hydrocarbons are comprised of at least 25% by weight of unsaturated hydrocarbons selected from the group of alkenes having the formula CnH2n, and cycloalkenes having the formula CnH2nxe2x88x922, where n equals 4, 5, 6, 7, or 8. Preferably, the grounded surface is a grounded conveyor belt, and the step of consolidating the fibrous batt includes the step of compressing the batt between the conveyor belt and a collection roll in order to form a consolidated nonwoven sheet. It is further preferred that the spin fluid be comprised of between 8 and 25 weight percent, based on the total weight of the spin fluid, of the fiber-forming polymer. The spin agent preferably consists essentially of hydrocarbons comprised substantially exclusively of carbon and hydrogen atoms. More preferably, the spin agent consists essentially of unsaturated hydrocarbons selected from the group of alkenes having the formula CnH2n, and cycloalkenes having the formula CnH2nxe2x88x922, where n equals 4, 5, 6, 7, or 8.